Subaru forged 2.5 MISFIRE after new plugs fine with old plugs
 

Hello everyone,

In need of some help; i have a classic with a forged ej25 running approx 400 bhp 1.1bar boost 1000cc injectors walbro etc.

Its been parked up for a while now 6 months or so and took it out drives fine. Did the oils etc and been using it for a month and its been perfect. Boosts great! Thought i'd give it some new plugs and fuel filter yesterday and i changed em over , the original plugs ngk pfr7b platinum gapped to 0.6 and i got the exact same ones gapped them to 0.6mm and it started fine but as soon as it goes onto boost anyrhing over 0.5 bar it starts sputtering and misfiring!

If i stay below 0.5 bar its perfect to redline anything more and cel flashes and sputters! Took plugs out gapped to 0.7 and exactly the same! Then got my original plugs put em back in and it was perfect again??? Decided it was a bad batch of plugs so got a set of iridims 7 heat range and didnt gap them this time as they were 0.7 standard just checked them and put them in and same problem? Misfire on full boost! Gapped them 0.6 and still the same! Also with the iridiums it seems to idle alot higher at 1100rpm which is odd? After a 40/50 mile drive with the new plugs and the misfiring on boost i pulled them out and the tips were very white? Whats going on?

So again put the original back in and back to normal and boosting fine with no problems so have left the originals in car for the time being. With the original plugs its fine and after pulling them to check the tips they are nice and golden but both new sets seem to have had problems..

Whats going on!!!

Please help

No codes ? Double check coil pack connections may be ? Double check new plugs are same. Misfires are a mare, I chased one for 2 years, replacing everything; cleaned injectors and that cured it. Good luck

PFR7B is the wrong plug for a 2.5 I think? The 2.5 uses a longer reach plug such as ILFR7 off the top of my head?

Yes wrong plugs

Depends if its got 2.0 or 2.5 heads on it.

Try a smaller gap, go down to 0.5mm boost may be blowing the spark out.

Ive checked the coil packs, checked everything! Its got 2.0 heads.. The original plugs are pfr7b and there fine its just as soon as i put new plugs in it (and thrre exactly the same) it misfired and run horrible on boost?

The originals are gapped to 0.6 so gapped the new ones to 0.6 but still misfires on boost.. If it was the wrong plugs the originals wouldnt work but they work fine...

Dodgy plug? It can happen.


Did you tighten the end of the plugs on when they were fitted?

Did the new plugs come from a reliable source, a lot of Chinese NGK copies about.

I had a similar issue with a Forester, it was the plug gap. Got to remember that the plugs you removed have probably been in there for a while and the surfaces that the spark travels between will behave a little different than brand new plugs.

Just reduce the gap ever so slightly and see what happens. Worked for me.

From eurocarparts lads there genuine? I tried a normal set the first time and it was misfiring as soon as u hit boost? Then i binned em and got a fresh set of iridiums and exactly the same?

Ive just tried the iridiums in my brothers subaru and they work fine? But in mine they misfire its as if the car only likes the original plugs.

Im so lost!

Thanks for the replies everyone!

I will reduce gap and try again...

I'd go & buy a new set of plugs ( & not from ECP

It could well be a dodgy plug or plugs. In my biking days I bought a new set of NGKs for my NC30 and within a few miles I had a misfire. Same for my R1 years later. Removed them, old set back in and everything was fine. Depends how the were handled I guess too, someone has maybe dropped them.

But two sets bad?

If it's 2 sets then either you have damaged a coil pack on first removal/ refit or you have disturbed something etc etc

I thought it was just the one set that went bad?

Cant be coilpacks tho?

Original plugs in car its perfect.

Changed plugs for new set exactly the same as those that come
Out and misfire on boost? Engine light flashes on boost.

Took new plugs out and refitted old plugs and back to normal with no misfire...

Went and brought A 2nd set brand new of iridiums this time, fitted and misfire on boost again??

Removed them and refitted original set and its back to normal again?

Find it hard to beleive two brand new sets of ngk can be bad?

are the gaps the same on the original set and the 2 new sets ?

Yes the originals gapped to 0.6 so gapped em at 0.6 and 0.7 and still the same will try gapping to 0.5 and try again

I'd be looking at the coil packs then or checking the injectors maybe? Odd that it runs fine on the original plugs. No signs at all with original plugs?

Absoluetely perfect on original plugs?